There has been an optical element having a light-transmitting substrate made of glass or plastic, which has been subjected to a surface treatment to reduce the reflection of light on the surface. As this type of surface treatment, there is a method of forming a fine and dense concavo-convex shape (moth eye) on the surface of an optical element (see, for example, Non-Patent Literature 1).
When light passes through the surface of an optical element having a periodic concavo-convex surface, diffraction of light generally occurs, whereby the straight component of the transmitted light is largely reduced. However, when the pitch of the concavo-convex shape is shorter than the wavelength of the transmitted light, diffraction does not occur. For example, when the concavo-convex shape is rectangular as described below, an anti-reflection effect for light having a single wavelength corresponding to the pitch and depth can be obtained.
As moth eye structures produced by electron beam exposure, fine tent-shaped moth eye structures (pitch: about 300 nm, depth: about 400 nm) have been disclosed (see, for example, Non-Patent Literature 2). In the moth eye structures, high-performance anti-reflection characteristics having a reflectance of 1% or less can be obtained.
As moth eye structures produced by a complex method of a process of manufacturing a master of an optical disk and an etching process, bell-shaped and elliptical frustum-shaped moth eye structures have been disclosed (see, for example, Patent Literature 1). In these structures, anti-reflection characteristics close to those manufactured by electron beam exposure can be obtained.